SVT Focus...nice. I have one of those!
But I converted it over to the Euro only RS Focus model...and with the bulging fenders and gaping grill, and 225 tires, it's probably a step in the wrong direction as far as aero is concerned (I know for a fact the drag is higher than a regular Focus hatchback).

But having the engine tuned (for performance) Also gave noticeable gains in mileage.
There are a couple of really good tuners you can talk to on the popular focus forums which would be happy to sell you a flasher with a custom tune...One of which, offers free upgrade tunes if you happen to change somethings like throttle body, exhaust, filter, etc.

Lowering a vehicle will only decrease the frontal area a tiny bit - only the area of the wheels gets reduce by the reduction in height. If you drop the car an inch, then the width of two tires; say 8" X 1" X 2 = 16 sq in. The Cd is not likely to change one iota; or it might even increase the Cd because it compresses the air under the car somewhat.

If the vehicle frontal area is 25 sq ft and it has a Cd of 0.35 then the CdA is 8.75 sq ft. Take away the 16 sq in and the CdA is 8.71 sq ft - a net improvement of 5.76 sq in.

If you install video mirrors in place of both side mirrors, you reduce the frontal area by about 80 sq in *and* you reduce the Cd by say 0.03 or so. The resulting CdA is 8.482 sq ft - net improvement of 38.56 sq in which is 6.6X better than the lowering.

With a good grill block, smooth wheel covers, rear wheel skirts, a smooth bellypan, a partial Kamm back, and the video mirror swap, you could drop the Cd to 0.31 or so? That would be a CdA of 7.58 sq ft - a net improvement of 168.8 sq in which is 29.3X better.

I gotta say, less air going under the car doesn't make it any better when it comes to fuel economy...turbulance is turbulance, weather it's 5" or 3" it's not much of a difference...there is a bigger benefit if you can "clean up" the turbulance because when you look at the best, most aerodynamic cars ever made, arn't slammed to the ground

I am "taking that into account". Lowering a car too much actually increases the Cd because there simply isn't room for the air to move around, and with a typical rough and lumpy and crevassed underside, this can cause some additional drag. You want the air to flow easily and smoothly, and moving vehicle is "squishing" the air around and the lower the vehicle, the less space there is for this to happen.

With a smooth underside then air is not as turbulent and the reduction in Cd is way greater than the reduction in area from lowering. Ideally, the rear of the underside is a slightly rising slope, and the ideal minimum is about 6-8" of clearance. Less clearance compresses the air more and causes lift - which is drag. A air dam adds frontal area and increases drag; though it reduces lift or even adds downforce - which is also drag. A smooth underside negates the need for an air dam and it reduces turbulence.

A smooth belly pan will be a much bigger improvement than lowering the car.

Edit: to emphasize GRU's point, look at the Schlörwagen. In the Hucho book, they have a chart showing the Cd at various heights - the *higher* it was the Cd went down (but this adds to the frontal area, and there was exponentially diminishing returns above a certain height). Lowering the car added Cd - more so with a rough underside. The best compromise with a smooth underside was with about 6" of minimum clearance.

Have you looked at the Hucho book? Would lumps and bumps on the sides or top be good for drag? Would shielding lumps and bumps on the sides or top help anything? The ground and the spinning wheels makes the underside into a special case, and turbulence there is amplified by the proximity to the ground.

If there is adequate ground clearance and the underside is sloped up, then this allows the air to flow back under the car, and the pressure at the back bumper can be a closer match to the sides and the top. This greatly reduces turbulence in the back - which is the single greatest cause of drag.

An air dam that is lower than any part of the underside greatly increases drag. An air dam is a kludge solution for lift and/or a rough underside.

During the few years I was making my pathetic attempts at Bonneville Salt Flats, I had the extreme pleasure of becoming friends with Tom Burkland, former piston powered, wheel driven land speed record holder at Bonneville (417 mph in a twin engined, 4 wheel drive streamliner, reaching 450 mph in one pass - an all time, piston powered, wheel-driven record speed). When discussing my poor pathetic efforts with him, his advice was to chose:

either slam the car down as close to the ground as you can (with a front air dam that also almost drags the ground) - the idea being to try to keep as close to ALL the air out from under it as you can,

or;

streamline the underneath with a belly pan just as smooth and aero as the top & sides.

Now, at Bonneville, there are rules out the ying-yang, and as I soon discovered, all the stuff that would REALLY help was also illegal! The car I was driving (actually, the CLASS I was in) did not allow belly pan streamlining. Therefore, the consensus was to get it as low to the ground as I could. HOWEVER, Tom's streamliner was allowed to have streamlining anywhere and everywhere, so he did the belly pan. (and it was pretty low, too). As I recall, if you have a choice, do the belly pan. Lowering is just the best you can do if you're not allowed a belly pan.

So after all that, I think I would have to concur with those who say "do the steamlined belly pan & you'll get more". Lowering should help too, but it's a weak, second-place choice. (The car in the Hot Rod article was in a class similar to mine, where belly pans were NOT allowed, too)

This same Tom Burkland I just mentioned (who also happens to be an aeronautical engineer in his "day job"), designed and built a "competition coupe" before he got into the streamliner project. Interestingly enough, this old competition coupe was bought by a guy here in Carson City who still races it. When I learned the aerodynamic design of it (and I was allowed to lay down next to it and look at the bottom), it just blew my mind. Tom designed it as an upside down wing!! It's higher from the ground at the tip of the nose, then slopes down closer in an airfoil curve to a minimum somewhere underneath, then back up towards the tail. Tom's theory; downforce without drag (or any more drag than a wing).

I just couldn't believe it. "What about the air you're compressing underneath?!?!?! You've GOTTA be causing LIFT!" Not so. The air accelerates under the car just like it would over a wing, producing a low pressure area under there. "But what about the air 'trapped' between the car and the ground?" I asked. The answer: no air is "trapped" - it's flowing under there, and the ground is just the opposite side of the venturi throat. It has to accelerate, causing the pressure to DROP. At about that time, my poor little brain began to smoke and I had to stop thinking!!!!

But it worked. Dan (Dan Webster, the car's current owner) routinely holds multiple land speed records with that car powered by vintage flathead Fords. I think he's right at 200 mph. And when Tom Burkland was racing it, he was hitting 300 mph with small block Chrysler engines. Weird, huh?

So the lesson I learned: Aerodynamics is a lot more complex than I thought it was!! LOL!!

This same Tom Burkland I just mentioned (who also happens to be an aeronautical engineer in his "day job"), designed and built a "competition coupe" before he got into the streamliner project. Interestingly enough, this old competition coupe was bought by a guy here in Carson City who still races it. When I learned the aerodynamic design of it (and I was allowed to lay down next to it and look at the bottom), it just blew my mind. Tom designed it as an upside down wing!! It's higher from the ground at the tip of the nose, then slopes down closer in an airfoil curve to a minimum somewhere underneath, then back up towards the tail. Tom's theory; downforce without drag (or any more drag than a wing).

I just couldn't believe it. "What about the air you're compressing underneath?!?!?! You've GOTTA be causing LIFT!" Not so. The air accelerates under the car just like it would over a wing, producing a low pressure area under there. "But what about the air 'trapped' between the car and the ground?" I asked. The answer: no air is "trapped" - it's flowing under there, and the ground is just the opposite side of the venturi throat. It has to accelerate, causing the pressure to DROP. At about that time, my poor little brain began to smoke and I had to stop thinking!!!!

But it worked. Dan (Dan Webster, the car's current owner) routinely holds multiple land speed records with that car powered by vintage flathead Fords. I think he's right at 200 mph. And when Tom Burkland was racing it, he was hitting 300 mph with small block Chrysler engines. Weird, huh?

So the lesson I learned: Aerodynamics is a lot more complex than I thought it was!! LOL!!

lol, that's called ground effects. The effect is more pronounced because the "bump" gets very close to the ground, and the air doesn't have anywhere to go, whereas on a wing there's the rest of the sky in place of the rigid ground.